The Velocity of Light*

Abstract

THE correct determination of the velocity of light is a result on which so much in physical science depends that there is good reason for us to give a description of the details of the apparatus used for the purpose of obtaining the exact value. Until the time that M. Cornu undertook experiments with this object in view the generally received value of the velocity in question was 298,000 kilometres per second. This depended on the experiments of M. Foucault, who used a rotating mirror on which the rays of light from cross-wires fell, and while the minor was in a certain position were reflected by it to a concave mirror at a distance of 13(1/2 feet, having the revolving mirror at its. centre of curvature and so fixed as to return the rays of light to the latter, which reflected them to the point of departure. While however the rotating mirror was in rapid motion, a ray of light reflected by it to the distant mirror and back from it, would not, unless the passage of light were instantaneous, reach the rotating mirror until the latter had moved from its position of first reflection, and would not therefore return to the point of departure, but to some point near it, depending on the angle through which the rotating mirror had moved in the time between its reflecting the ray to the concave mirror and the return of that ray, By placing the cross-wires in the principal focus of a convex lens the rays proceed in a parallel beam, and on returning form an image of the wires, removed from the wires themselves, a distance depending on the angular velocity of the mirror and the velocity of light The cross-wires and their images are rendered visible by viewing them by means of a diagonal reflector of plain glass in front, which at the same time allows sufficient light to pass through to illuminate them.²